1. Field of the Invention
The present invention relates to a sheet processing apparatus having a buffer function for retaining following sheets during execution of post processing, such as binding, on sheets having undergone print processing, an image forming apparatus, and a sheet buffering device.
2. Description of the Related Art
Conventionally, there has been proposed a sheet processing apparatus that performs processing, such as conveyance, stacking, and sorting, on sheets discharged from an image forming apparatus or the like. The sheet processing apparatus has a conveying path provided therein with a section for temporarily side-tracking a sheet, and temporarily stops conveyance of the sheet at the side-tracking section (buffer holding operation) and then superimposes the sheet on another sheet conveyed from upstream and conveys the sheets superimposed one upon the other (buffer conveying operation).
It has been proposed to secure time required for sheet processing performed downstream of the side-tracking section in the sheet conveying path by the buffer conveying operation, to thereby avoid degradation of the overall processing efficiency of sheet processing. Hereafter, the buffer holding operation and the buffer conveying operation will be generically referred to as “the buffer processing”.
In the prior art, in a case where the buffer processing is performed by superposing a plurality of sheets one upon another into a buffered bundle, the buffered bundle is conveyed with the leading edges of the superimposed sheets shifted from each other (see e.g. Japanese Patent Laid-Open Publication No. 2001-097631). Specifically, timing for superposing sheets one upon another is determined based on detection of the leading edge of each of the sheets. The sheets are shifted from each other such that a leading-page sheet is positioned most upstream in a sheet conveying direction and following-page sheets are sequentially shifted downstream in the sheet conveying direction, so as to facilitate alignment of the trailing edges of the superimposed sheets stacked on a processing tray.
However, in a case where the buffer processing is performed based on the leading edge of each sheet as described above, when a particular sheet, such as a tab sheet, having a non-straight leading edge is buffered, a sensor for detecting the leading edge of a sheet can detect the tab portion of the tab sheet as the leading edge of the same, and hence there is a fear of the sheet being improperly shifted.
In the sheet leading edge-based buffer processing, the sheet shift direction is determined in consideration of alignment of sheets superimposed into a bundle, such that a leading-page sheet is positioned most upstream in the sheet conveying direction and following-page sheets are sequentially shifted downstream in the sheet conveying direction. In doing this, the leading edge of a sheet is detected differently by the sensor for detecting the leading edge of a sheet being conveyed, depending on whether the sensor detects the tab portion of a tab sheet or the edge of a sheet other than a tab sheet. Consequently, the shift direction of the trailing edge of a sheet relative to another sheet can become different between the above-mentioned two cases.
As a solution to this problem, it can be envisaged that a plurality of sensors are arranged in a lateral direction orthogonal to the sheet conveying direction to thereby reliably detect a sheet edge other than a tab portion for execution of the buffer processing. However, since it is required to provide the plurality of sensors, increase in cost cannot be avoided.
Thus, even when a tab sheet or the like sheet having a non-straight leading edge is to be subjected to the buffer processing, it is required to execute superposition control which makes it possible to ensure the relationship with another sheet in the direction of shift of the trailing edge of the tab sheet from the other sheet, without using a plurality of sensors.